In recent years, the importance of the environmental performance is increasing, and needs for reducing the weight of rubber articles or tires in which a steel cord is used for a reinforcing member are increasing. In order to reduce the weight of a tire, it is effective to reduce the thickness of a belt. In order to reduce the thickness of the belt, it is effective to make the cross-section of the steel cord into a flat shape, and to make the diameter (minor axis) of the steel cord small.
In general, when a steel cord having a (1×N) structure is made into a flat shape by using a jig such as a pressing roller, an effect of a reinforcing member is reduced since initial elongation occurs when a tensile input is applied to the steel cord. Accordingly, as a cord structure which has a flat shape without compromising the tensile rigidity of the steel cord, for example, structures described in Patent Documents 1 to 4 are proposed. In the steel cord described in Patent Document 1, a technique is proposed in which a 2+6 structure is employed and by providing a space between sheath filaments, rubber penetrates between the filaments. A steel cord described in Patent Document 2 tries to overcome the above-mentioned problems by defining the diameter of a core filament and the diameter of a sheath filament. Further, in the steel cord described in Patent Document 3, a technique is disclosed in which, by defining the diameter of a core filament and the diameter of the sheath filament, rubber penetration to a steel cord is improved. Still further, in Patent Document 4, a technique is disclosed in which, by employing a single-twisted structure or a layer-twisted structure of 6 to 10 steel filaments, rubber penetration is ensured is ensured.
Particularly in a tire for heavy load, there is a trend of large size in the tire, and demands for improving a variety of performances such as steering stability or durability are increasing. In general, as the tire becomes large in size, growth of the diameter of the tire becomes large when the tire is inflated to an inner pressure, which affects significantly on the performance or durability of the tire. For this reason, in a large tire, the growth of the diameter of the tire is inhibited by using a reinforcing layer which is arranged in substantially tire circumferential direction, in other words, in parallel to the tire equatorial line.
As an improvement technology of such a tire for heavy load, it is reported, for example, in Patent Document 5 that by having a predetermined relationship between the minor axis of steel cord bundle and the bundle width thereof and further satisfying a predetermined relationship between the bundle width of the steel cord bundle and the bundle interval, steering stability and durability can be improved while reducing the weight of a tire.